1. Field of the Invention
The present invention relates to the field of flat display screens, and more specifically to a cathodoluminescent screen, the anode of which supports phosphor elements likely to be excited by electron bombarding. This electron bombarding may originate from microtips, from low extraction potential layers, or from a thermo-ionic source. To simplify the present description, only microtip screens will be considered hereafter, but it should be noted that the present invention generally relates to the various above-mentioned types of screens and the like.
2. Discussion of the Related Art
In a microtip screen, a so-called cathode plate is provided with electron emission microtips and is arranged to face a so-called anode plate provided with phosphor elements. The cathode is associated with a grid provided with holes corresponding to the locations of the microtips. This device uses the electric field which is created between the cathode and the grid to extract electrons from the microtips. These electrons are then attracted by the phosphor elements of the anode if said elements are properly biased.
The present invention more specifically relates to a cathode of a flat display screen associated with at least one so-called extraction grid, that is, a cathode plate.
The microtips are generally deposited on cathode conductors organized in columns that form active electron emission areas. The columns are addressable individually. The extraction grid is organized in rows perpendicular to the cathode columns, also addressable individually. In a color screen, the anode is for example provided with alternate strips of phosphor elements, each corresponding to a color (red, green, blue). The strips are then generally parallel to the cathode columns and can be separated from one another by an insulator. The phosphor elements are deposited on electrodes formed of corresponding strips of a conductive layer, for example made of indium and tin oxide (ITO) for a transparent anode. In a monochrome screen, the anode supports a plane of phosphor elements of same color or two separately addressable sets of phosphor elements of same color, for example, organized in alternate strips as in a color screen. The intersection of a cathode column and of a grid row defines a screen pixel. For a color screen, the sets of red, green, blue strips of the anode are often alternately biased with respect to the cathode so that the electrons extracted from the microtips of a pixel of the cathode-grid are alternately directed towards the phosphor elements of each of the colors. In some color screens, the intersection of a grid row with a cathode column then defines a sub-pixel of a color. In other screens, the pixels may be defined individually by elementary patterns of phosphor elements of each color on the anode side, these chips then being addressable, for example, by groups of same color.
In some screens, the anode, while being formed of several sets of strips or of elementary patterns of phosphor elements, is not switched by set of strips or patterns. All strips then are at a same potential. The anode is then said to be unswitched, as opposed to so-called switched anodes where the colors are sequentially biased.
Generally, the grid rows are sequentially biased to a potential on the order of 80 volts, while the strips or sets of phosphor elements to be excited are biased under a voltage of several hundreds, or even several thousands of volts, via the ITO strip on which the phosphor elements are deposited. In the case of a switched anode, the ITO strips supporting the other strips of phosphor elements are at a low or zero potential. The cathode columns are brought to respective potentials ranging between a maximum emission potential and a no-emission potential (for example, respectively 0 and approximately 40 volts). The brightness of a color component of each of the pixels in a line is thus determined. The choice of the values of the biasing potentials is linked to the characteristics of the phosphor elements and of the microtips. Conventionally, below a potential difference of approximately 50 volts between the cathode and the grid, there is no electron emission, and the maximum electron emission used corresponds to a potential difference on the order of 80 volts.
The manufacturing of microtip screens uses the techniques currently used in integrated circuit manufacturing. In particular, the cathode and the grid are generally formed of thin layer depositions on a substrate, for example made of glass, forming the screen bottom. The anode is generally formed on another glass substrate forming, in this example, the screen surface. The anode and the cathode-grid are formed independently from each other on the two substrates, then are assembled by means of a peripheral seal, while leaving, between the grid and the anode, an empty space to enable circulation of the electrons emitted by the cathode to the anode. Once finished, the internal screen space is thus encircled by the seal, generally made of glass, which seals the anode and cathode plates. This seal must be placed distant from the active areas of the anode and of the cathode, in particular to enable the necessary interconnections of the elements. Reference will be made hereafter to the active screen area, be it on the cathode-grid side or on the anode side. A space is generally left between this active area of the anode and of the cathode and the peripheral seal. This space is most often made of an insulating material, for example, silicon oxide due to the use of technologies derived from those used in integrated circuit manufacturing.
A problem which is posed in conventional screens is the occurrence of destructive phenomena due to the forming of arcs at the periphery of the screen or of its active area. Such phenomena are due to the developing of a charge area at the periphery of the active area in the insulating space separating said area from the sealing wall. This charge area also propagates at the seal surface and thus progressively comes closer to the other electrode.
This positive charge area is generated by electrons emitted towards the anode during screen operation, and which fall back on the insulating areas at the edge of the active area. The developing of this positive charge area is self-fed by the fact that the more the positive area increases, the more it attracts new electrons. This charge area ends up causing either arcs between the screen edge and the cathode electrodes, or a parasitic emission phenomenon.
The present invention aims at overcoming the disadvantages of conventional screens.
A feature of the present invention is to provide, on the cathode-grid side, a peripheral protection area between the active area, that is, the surface participating in the display, and the peripheral sealing wall. This peripheral protection area, formed of at least one conductive section, has the function of preventing the propagation of secondary electrons to the sealing wall by trapping the electrons that fall back on this or these sections. The conductive section(s) occupy, in a peripheral pattern of the active area, a sufficiently large perimeter to make the secondary electrons likely to cross the barrier thus formed negligible. Across the width (between the active area and the closest portion of the sealing wall), the conductive section(s) cover a distance greater than the distance that most secondary electrons that may be emitted are likely to cross. This distance depends on the energy of these secondary electrons, which itself depends on the energy of the primary electrons and on the inter-electrode space. For a given sizing and given operating conditions, it is known to statistically determine the energy distribution of the secondary electrons, and thus the energy of the statistic majority of secondary electrons.
According to first embodiment, the protection area is formed of at least one conductive ring formed in a layer deposited, with an interposed insulating layer, on the so-called extraction grid. In screens where an additional grid (for example, a focusing grid) is provided on the extraction grid, the peripheral conductive ring can be formed in the layer of formation of this additional grid, at the periphery of the active area.
According to a second preferred embodiment, the peripheral protection area is formed in at least one of the conductive levels from among the level in which the cathode conductors are formed and the level in which the extraction grid is formed.
This preferred embodiment has several aims.
A first aim is that the forming of the protection ring results in no additional complexity in the cathode-grid manufacturing.
Another object of the present invention is to introduce no additional manufacturing step in the method of forming a flat display screen cathode plate.
Another object is to solve problems specific to the cathode-grid plate.
Indeed, the grid rows and the cathode columns are addressed individually. They thus require a large number of conductive sections on two edges of the cathode plate that risk being hampered (mechanically or functionally) by a peripheral conductive ring. As a comparison, on the anode side, only three conductors come out for a color screen since the sets of strips are generally addressed simultaneously per color.
More specifically, the present invention provides a cathode plate of a flat display screen of the type including a set of electron emission cathode conductors, organized in columns, a set of electron extraction grid conductors, organized in rows, and a peripheral protection area, surrounding an active area taking part in the display, to prevent propagation of secondary electrons out of the perimeter of the protection area.
According to an embodiment of the present invention, the peripheral protection area is formed of a conductive ring surrounding the majority of the active area and formed in an accessible conductive level.
According to an embodiment of the present invention, the cathode plate includes, on either side of the extraction lines, at least one additional accessible conductive line.
According to an embodiment of the present invention, the cathode plate includes, on either side of the electron emission columns, at least one additional conductive column.
According to an embodiment of the present invention, the grid lines and the cathode columns belong to a piling of thin layers with at least one interposed insulating layer, the additional line(s) and/or column(s) being formed in the respective levels of the extraction lines and of the emission columns.
According to an embodiment of the present invention, the additional column(s) are at least partially accessible.
According to an embodiment of the present invention, the cathode emission columns extend under the additional line(s), the grid extraction lines extending over the additional column(s).
According to an embodiment of the present invention, the additional column(s) are adapted to being biased to a potential corresponding, for the emission columns, to no electron emission, the additional line(s) being adapted to being biased to a potential corresponding, for the extraction lines, to no addressing.
According to an embodiment of the present invention, the number of additional columns and/or lines is a function, in particular, of the column and line conductor width and on the angle of the electron emission cone of the cathode.
The present invention also provides a flat display screen including a cathode provided with active electron emission regions, a cathodoluminescent anode provided with at least one active area of phosphor elements, and a grid for extracting electron emitted by the active regions of the cathode towards the phosphor elements, the cathode and the grid being formed on a cathode plate of the present invention.
According to an embodiment of the present invention, the number of additional columns and/or lines depends, along others, on the distance separating the cathode plate from the cathodoluminescent anode.
According to an embodiment of the present invention, the screen includes a circuit for biasing and addressing the different conductors of the cathode, of the grid and of the anode, provided with connections for biasing the additional lines and/or columns.
The foregoing objects, features and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments, in conjunction with the accompanying drawings.